This invention pertains generally to the field of light sources and particularly to plasma discharge light sources.
Many types of light sources have been investigated and to some extent commercially developed in an effort to improve upon the efficiency of the traditional incandescent lamps. While incandescent lamps provide light which has a satisfactory xe2x80x9cwhitexe2x80x9d spectrum as perceived by human observers, the efficiency of the incandescent lamp in converting electrical energy to luminescence in visible wavelengths is relatively low. Commercial alternatives to incandescent lamps which have a higher efficiency in conversion of electrical energy to visible light include tungsten-halogen lamps, fluorescent lamps, metal halide lamps, high pressure sodium lamps, and low pressure sodium lamps. The development of alternative lighting sources continues to be an important area of research for several reasons. First, lighting accounts for approximately 20 to 25 percent of the total electrical power consumption in the United States. Second, there has been little increase in the amount of white light yielded per watt of power during the last 15 to 20 years, due, in part, to the numerous requirements placed on light sources. For example, sodium lights are efficient, but produce a monochromatic yellow-orange color. While incandescent bulbs produce an acceptable white light, they are only {fraction (1/10)} as efficient as low pressure sodium lights. A third factor is a demand for a substitute for fluorescent light technology. Fluorescent lights contain mercury, a well-known bioaccumulative neurotoxin. Currently, there are no practical substitutes for mercury-based white light technology.
In accordance with the present invention, visible light emission is obtained from a plasma containing elemental barium. Elemental neutral barium provides a strong green light emission in the center of the visible spectrum with a conversion of electrical energy in the plasma discharge into visible light. In accordance with the invention, by the selective excitation of barium ionic species in the plasma discharge, emission of visible light at longer and shorter wavelengthsxe2x80x94red and bluexe2x80x94can be obtained simultaneously from the plasma discharge with the green emission from neutral barium, effectively providing light that is visually perceived as white. Appropriate selection of the plasma conditions allows selection of the perceived coloration of the light from the source.
A device for emitting visible light in accordance with the invention comprises a discharge vessel which contains elemental barium and a buffer fill gas, which is preferably a noble gas (e.g., argon, helium, xenon, krypton and neon). A discharge inducer is coupled to the discharge vessel to induce a desired temperature and a desired barium vapor pressure in the discharge vessel, producing barium vapor which mixes with the fill gas. The discharge inducer excites a plasma in the barium vapor and the fill gas and induces visible light emission from the plasma. The discharge inducer may comprise a DC or AC power source coupled to electrodes within the vessel to provide an electrical discharge across the electrodes and thereby induce the discharge in the fill gas and in the barium vapor. The discharge inducer may also comprise a coil coupling radio frequency (RF) power from an RF power source inductively to the plasma within the vessel. The discharge inducer may also comprise a capacitive coupler for coupling RF power to the plasma.
To maintain the temperatures required for barium vaporization within the discharge vessel, thermal insulation may be provided around the discharge vessel. Such thermal insulation may comprise a partial jacket of insulating material or a thermal vacuum jacket of transparent glass which is evacuated between the jacket and the discharge vessel.
Appropriate selection of the mixture of neutral barium atoms and barium ions in the discharge plasma may be utilized to select the perceived color of the light emission. Neutral barium atoms in the plasma emit at wavelengths centered at 5535 Angstroms, while barium ions emit at wavelengths centered at 4934, 4554, 6141 and 6496 Angstroms. Thus, by appropriate selection of the mixture of ions and neutral barium atoms in the plasma, a desired balance of emission wavelengths may be obtained, including light that is perceived as substantially white.
In the method of carrying out the invention to provide visible light, the discharge vessel with fill gas and an elemental barium dose therein (e.g., a block of pure elemental barium) may be provided, with power then being applied from a power source to the discharge vessel to increase the temperature of the gas fill and the barium to achieve a desired operating temperature to cause the barium to vaporize. The vapor and fill gas are excited to produce a visible light discharge emission from the discharge vessel. The pressure within the discharge vessel or power and temperature may be adjusted to adjust the visible light discharge emission and to select a desired color hue for the emission. Energy may be provided to excite the plasma by an electrical discharge across electrodes in the discharge vessel, by inductively coupling radio frequency power to the plasma, by capacitively coupling radio frequency power to the plasma, or in other conventional manners.
Further in accordance with the invention, a method of converting electrical energy to visible light comprises enclosing a source of elemental barium in a sealed vessel with a fill gas at a desired pressure, exciting the gas within the vessel to heat the barium to evaporate barium atoms and to provide a barium vapor in the vessel, applying energy to the barium vapor to provide a plasma comprising a mixture of neutral barium and barium ions, and to excite emission from the elemental barium and the barium ions to emit light at wavelengths characteristic of the neutral barium and barium ion species. The energy may be applied to the plasma by applying an electrical discharge across electrodes within the vessel or by coupling radio frequency energy into the plasma capacitively or inductively.
In accordance with the invention, the fill gas utilized within the discharge vessel may comprise one of the noble gases without the necessity for utilizing a mercury vapor. Because mercury is not required, the problem of disposal that is incurred with conventional light sources which require mercury, such as fluorescent lights and various types of mercury lamps, can be avoided.
Further objects, features and advantages of the invention will be apparent from the following detailed description when taken in conjunction with the accompanying drawings.